In electrographic printers, discretely controlled ionization is employed to form the charged image on a dielectric surface. The dielectric surface then passes through a developer unit for developing a latent image thereon, and the developed electrostatic latent image is then transferred to a sheet of copy material (typically paper). Uniform and consistent ion formation and transport in the electric field is essential to guarantee the quality of the imaging, however contamination often readily occurs.
Ionization is typically accomplished by selectively energizing electrodes in an electrode array. During printing, only a very small proportion of the electrodes will be energized at any one time. Contamination of the exposed electrodes occurs by the deposition of polymers and aerosols that are present in the surrounding environment, and by the deposition of compounds generated in the ion imaging process itself which result from the ionization of gas in the presence of other contaminants. Subtle degradation of the electrodes can also occur due to the migration of non-precious metals through precious metals protective electrode coatings.
As a result of the build-up of contaminants on the electrodes, the quality of the print can decrease remarkably, and in fact the contaminants may ultimately provide a total insulative blockage of the electrode surface due to a microscopically thin layer that is not readily detectable by visual examination. This latter effect is particularly insidious since the "Townsend Multiplication" phenomena results in regeneration of charge carriers that become unstable resulting in infrequent, but noticeable, imaging defects.
Because of the reduction in print quality that occurs from electrode contamination, methods of cleaning the electrodes have been suggested, such as in U.S. Pat. No. 4,638,339 (the disclosure of which is hereby incorporated by reference herein). Such cleaning systems typically comprise wiping or otherwise physically acting upon the electrode and/or dielectric belt surfaces to remove contaminants during a non-print portion of the cycle of operation. Such procedures are much less effective than desirable, however, and either do not produce the desired results or introduce problems of their own.
According to the present invention, a method of electrostatic cleaning of the ion electrodes of an electrographic printer is provided which is very effective, and which is very easy to implement and control, and results in a minimum disturbance of the printing cycle. Basically, according to the invention electrographic printing is periodically temporarily interrupted, and then substantially all of the electrodes in the electrode array are energized for a period of time sufficient to effect cleaning of the electrodes. While the invention is applicable to a wide variety of electrographic printing structures and procedures, it is particularly useful with respect to electrographic printing elements as disclosed in said U.S. Pat. No. 4,638,339 since the relative positioning, and the construction, of the components there (with sufficient gap to support Townsend Multiplication) provides a very effective "burn off" of the contaminants.
While the cycles of interruption and energization can vary widely depending upon the particular circumstances, typically printing would be interrupted after the printing of every 100 pages, and substantially all of the electrodes would be energized for a time period equal to the approximate time it takes to print about four pages. By the practice of such a technique, not only can a clean print head be kept clean, the print quality of a contaminated print head can be brought back to acceptability over a period of time. For example, a moderately contaminated print head can be brought back to acceptability after about 300 pages of print, whereas a badly contaminated head would reach acceptability after about 2,000 pages of printing.
It is possible to practice the invention with only a minimum of equipment and minimum modifications of existing equipment and techniques. For example, when employed with an electrographic printer such as shown in said U.S. Pat. No. 4,638,339 with a host computer controlling the printer functions, including the selective energization of the electrodes all that is necessary is a simple, straight-forward change in the software which any programmer can readily implement. The software changes would typically provide for periodic temporary interruption of the printing (preferably after the count of the number of printed pages), movement of the developer away from the dielectric belt, energization of substantially all of the electrodes (pins), and then after energization of substantially all of the electrodes for a predetermined period of time, restarting the electrographic printing procedure.
It is the primary object of the present invention to provide for the effective yet simple cleaning of an electrographic print head. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings and from the appended claims.